Individuals living with a lower extremity amputation (LEA) often experience relative motion between their residual limb and the prosthetic socket, such as vertical translation and axial rotation. This motion causes inefficient dynamic load transmission from the distal prosthetic components to the residual limb, which can lead to significant secondary consequences, such as pain, gait deviations, and discomfort that limit mobility and autonomy. Over time, inefficient load transmission can lead to elevated forces on the intact joints, which can result in higher risk and incidences of degenerative diseases. There is a substantial gap in our understanding of the complex mechanics of the residual limb-socket interaction during dynamic activities that limit the ability to improve prosthetic design. Although assessments of the relative motion between the bone and the prosthetic socket have been performed, currently there is little existing data on dynamic, in vivo residual limb-socket kinematics. Dynamic Stereo X-ray (DSX) is the only currently available technology that can achieve sub- millimeter bone pose (position and orientation) estimation accuracy during a wide variety of functional movements, but current analytical methods and tools often rely on subjective input and are extremely time consuming. DSX is a 3D imaging technology for visualizing rapid skeletal movement in vivo. DSX combines 3D models of bone morphology derived from computed tomography (CT) scans (required to generate the subject specific bone models of the remnant femur for tracking skeletal kinematics) with movement data from biplanar x-ray video to create highly accurate re-animations of the bone moving in 3D space. It allows for the calculation of joint angles and range of motion (ROM) during activity. Utilizing DSX, our 2 year goals for this pilot project are to develop and validate time-efficient 3D quantitative functional assessment tools to quantify the in vivo kinematics between the residual limb and prosthetic socket, in 6 degrees of freedom (DOF) of motion for individuals with transfemoral amputation. To verify the analytical tools and their relevance to TFA, we will evaluate two socket designs: a traditional encapsulated socket and a Compression/Release Stabilization (CRS) socket. To do so, the investigators will address the following aims: (1) To quantify, in 6 degrees of freedom of motion, the relative motion between the residual bone and the prosthetic socket during dynamic activities using DSX; (2) To compare comfort, quality of life, satisfaction, perceived stability, and ease of use of two lower limb socket designs. To address these aims, 5 subjects with TFA will be randomly assigned to start the study with their traditional, encapsulated socket or a fabricated CRS socket. Each subject will wear the assigned socket for 4 weeks of home use. After 4 weeks, the process will be repeated with each subject utilizing the second socket. After each period of home use, subjects will be administered the Trinity Amputations and Prosthetics Experience Scale (TAPES) satisfaction scale, and items related to socket comfort and fit drawn from both the Prosthetic Evaluation Questionnaire (PEQ) and Prosthetic Profile of the Amputee (PPA). Furthermore, a qualitative assessment will be performed through semi-guided interview. Following 8 weeks of home use, each subject will then be transported to Providence, Rhode Island (Brown University), where a CT scan will be performed and DSX will be utilized to record dynamic X-ray sequences during walking at self-selected speed, fast walking (10% faster), and sudden stop. Gait and movement data will be collected simultaneously with the XROMM during each dynamic task. By developing the analytical tools for a highly accurate in-vivo assessment of residual limb-socket motion, we can provide vital foundational information to aid in the development of new methods and techniques to enhance prosthetic fit that have the potential to reduce secondary physical comorbidities and degenerative changes that result from complications of poor prosthetic load transmission.